We propose experiments which test the hypothesis that 1) a major role of the choroidal circulation is to function as a heat sink for dissipation of light-induced heat, 2) that there exists a centrally mediated reflexive control of choroidal blood flow which responds to light or light-generated hear, and 3) that dysfunction in the ability of the choroidal circulation to stabilize the temperature of the retina may be a major environmental parameter to be considered in macular and other retinal disorders. We propose to 1) measure changes in tissue temperature using a thermistor probe inserted into the retina and choroid of the macular area of anesthetized primates, while varying ambient illumination and chorodial blood flow (by varying intraocular pressure), 2) study the reflexive control of chorodial blood flow by a) measuring changes in tissue temperature in the macular area and on the scieral surface after stimulating the contralateral eye with light and b) directly measure changes in blood flow produced by light stimulation of the contralateral eye using hydrogen washout techniques, 3) determine whether the effective stimulus for producing a reflexive change in chiroidal blood flow is light or light-induced heat by directly measuring choroidal blood flow and tissue temperature in response to a thermal stimulus to the contralateral eye applied with either long infra-red radiation or warming through an intraocular thermocouple, 4) determine the neural pathways and pharmacologic sensitivities mediating this reflex in primates by selective ablations and application of agonist and antagonist drugs, 5) measure changes in scieral surface temperature in response to stimulation of the contralateral eye with light of different intensities and various wavelengths in normal human volunteers, and 6) measure changes in scieral surface temperature in response to stimulation of the contralateral eye with light of different intensities and wavelengths in patients with various macular and retinal disorders.